1. Field of the Invention
The present invention has for its object processes for manufacturing mirror substrates made of resin concrete and the mirrors obtained by these processes.
The technical sector of the invention is that of the manufacture of mirrors.
2. Description of the Prior Art
Traditionally, mirrors are manufactured from a block of glass of which one face is machined and polished with very high precision along a determined geometrical surface which may be plane, spherical, parabolic or any other convex or concave geometrical shape.
The quality of the images depends on the variations between the real reflecting surface and a theoretical geometrical surface.
A so-called .lambda./10 quality astronomical mirror must present a reflecting face of which the variations with respect to the theoretical geometrical surface are in all points less than one tenth of the average wave-length of the visible radiations to be reflected.
The works for manufacturing and polishing a traditional .lambda./10 quality glass mirror with a diameter of 0.5 m, last about a month and a half and the cost of such a mirror is included between 50,000 and 100,000 francs. The manufacture of a conventional astronomical .lambda./10 quality mirror having a diameter of 1.50 m lasts about six months and the cost is included between 800,000 francs and 1.6 million francs.
In order to reduce the costs and durations of manufacture, it has been sought to manufacture mirrors by so-called surface replication processes consisting in making a glass mould of which one face is polished with very high precision along a conjugate surface or one complementary of the reflecting face to be made, and in casting into this mould a layer of polymerizable resin of which the face placed in contact with the polished face of the mould is perfectly smooth and reproduces the impression thereof.
The outer face of the layer of resin is then coated with a reflecting layer. The reflecting layer may be deposited on the polished face of the mould before the layer of resin is applied thereon in accordance with the technique described in Pat. No. FR. 1.187.363 (F. SCHWARTZ-KINO).
This so-called surface replication technique makes it possible to use the same mould a very large number of times in order to manufacture series of identical mirrors, but it must be avoided that the polished face of the mould be deformed or scratched.
Moreover, in order to manufacture large aperture mirrors of astronomical quality by this technique, the layer of resin must be supported by a rigid, undeformable substrate and manufacture of this substrate must not involve high costs in order that the cost of manufacturing the mirrors is less high than that of conventional mirrors.